1. Field of the invention
The invention relates in general to devices for influencing optical signals and to methods for producing them, and specifically to an optical lowpass filter for imaging or image receiving systems and a method for producing this optical lowpass filter.
2 . Description of Related Art
Optical lowpass filters are conventionally used, for example, for the sensors of digital cameras. A substantial task of an optical lowpass filter is to attenuate signal components that are above the Nyquist frequency. It is also advantageously the aim to use optical lowpass filters to reduce defective images such as moiré effects and aliasing.
Particularly in the case of integrated image sensors such as CCD and CMOS sensors, spatial frequency components that are above the limiting resolution of the sensor act in interfering fashion since these can lead to defective information at the location of the sensor. This results in image defects such as apparent resolution or contrast reversal (image noise, image artifacts).
Various types of optical lowpass filters are known in practice. For example, US 2004/0042079 (Osawa et al.) discloses birefringent optical lowpass filters. These can be configured as multilayer quartz plates.
Furthermore, JP 55074512 A (Michihiro) discloses an optical lowpass filter that is constructed from a multiplicity of optical elements whose refractive indices change from one side to the other.
The chief common feature of the known optical lowpass filters is that they can be expensive to produce and sensitive, above all, to mechanical loads. However, filters that consist of a number of layers are mostly relatively thick. Furthermore, filters made from birefringent quartz plates can lead to chromatic aberrations.
It is disadvantageous above all in the case of digital imaging systems that the sensitivity of the optical sensors can be strongly dependent on the incidence angle of the incoming light. This necessitates a telecentric alignment of the sensor.
Consequently, in order to even out these irregularities, but also to reduce defective images such as step formation, digital cameras mostly have an arithmetic unit in which the known image defects are removed by calculation mathematically. The result of this is that the resolution achieved frequently no longer corresponds to the resolution of the sensor. Moreover, removing errors by mathematical calculation by means of a microchip is expensive in terms of energy. Precisely in the case of high resolutions, there is a need for very high computing powers than can greatly shorten the service life of the digital camera. Again, the computing times are often so long that they result in annoying pauses between taking a picture and the renewed readiness of the apparatus for shooting.